1. Field of the Invention
The present invention relates to a process for forming a strontium-containing thin film by using a raw material suitable for forming a thin film containing strontium oxide or strontium sulfide by chemical vapor deposition (hereinafter, referred to as CVD) or atomic layer deposition (hereinafter, referred to as ALD).
2. Description of the Related Art
Films having high dielectric constant deposited by CVD and ALD such as films of SrTiO3, SrBi2Ta2O9, SrBi4Ti4O15are expected to serve as a dielectric for a highly integrated semiconductor device. A SrRuO3 film is discussed for its use as a ferroelectric film electrode.
Conventionally, as for a raw material for forming these strontium-containing films by CVD and ALD, bis(dipivaloylmethanato)strontium (Sr(C11H19O2)2; hereinafter, referred to as Sr(dpm)2) has been mainly discussed.
However, Sr(dpm)2 has a problem in its supply, because it has a very low vapor pressure of 0.1 Torr/231° C. due to trimerization thereof.
Further, since it undergoes thermal decomposition at 230° C. or higher, it has a problem of simultaneous occurrence of thermal decomposition beyond control with desirable self-limited growth in film-forming by ALD.
There is therefore a need for an organostrontium compound having higher vapor pressure, higher reactivity with anoxidizer, and higher thermostability.
Examples of a candidate compound include a known compound, bis(pentamethylcyclopentadienyl)strontium (Sr[C5(CH3)5]2; hereinafter, referred to as SrCP*2). SrCp*2 is not an adduct coordinated with diethyl ether ((C2H5)2O; hereinafter, referred to as Et2O) and tetrahydrofuran (C4H8O; hereinafter, referred to as THF).
These adducts have low thermal stability, release an added molecule by heating, are thermally denatured, and thus have unstable vapor pressure. These adducts also contain an oxygen atom, and can provide an oxygen atom through self-decomposition. These adducts are thus not preferred for use as the raw material in ALD.
In contrast, SrCp*2, which is not an adduct, is a monomer, has the highest vapor pressure among the organostrontium compounds, and immediately reacts with water as an oxidant. These properties are preferred for use as the raw material in ALD. In addition, since SrCp*2 has five methyl groups, it is more soluble in an organic solvent.
Therefore, the present inventors have disclosed a method for preparing SrCP*2 in Japanese Patent Application No. 2006-330359.
However, since SrCP*2 has a melting point of 207° C. and is in the solid state at room temperature, a sublimation step is required for final purification of SrCp*2 in the method. In addition, since SrCp*2 is a solid easily denatured by a trace amount of oxygen and/or water, it requires an expensive equipment and meticulous care.
Therefore, there is a need for a compound that can be purified by distillation, which is an effective purification method, and is in the liquid state at room temperature to 50° C. so that handling of the compound is easy under an inert atmosphere.
In other words, there is a need for a strontium compound that has a cyclopentadienyl group active to oxygen and water, does not contain ethers added thereto, present as a monomer, has high vapor pressure, has a group suitable for mass production, and is in the liquid state.
Now, a β-diketone based strontium complex such as the Sr (dpm)2 is synthesized with metallic strontium as a raw material. However, since several ppms of Na and K are contained in the metallic strontium, several ppms of Na and K are contained in a crude compound as well.
Furthermore, a cyclopentadienyl-based strontium compound such as SrCp*2 is synthesized with an alkali metal compound such as NaC5(CH3)5 (hereinafter referred to as NaCp*) or KC5(CH3)5 (hereinafter referred to as KCp*) as a raw material; accordingly, in a crude compound, Na or K is contained much.
Since the β-diketone based strontium complex and SrCp*2 are in a solid state around room temperature, these are difficult to purify by distillation, that is, Na and K derived from the raw materials are difficult to remove efficiently.
Accordingly, it has been difficult to obtain a raw material for forming a strontium-containing thin film containing Na and K each at a content of 50 ppb or less. That is, there has been no raw material for forming a strontium-containing thin film, which is applicable to CVD and ALD and contains Na and K each at a content of 50 ppb or less.
The present inventors have thought that bis(propyltetramethylcyclopentadienyl)strontium (Sr[C5(CH3)4(C3H7)]2; hereinafter, referred to as Sr(PrMe4Cp)2) is preferable, because it can be prepared by using tetramethyl(n-propyl)cyclopentadiene (C5(CH3)4 (C3H7)H) having an analogous structure to pentamethylcyclopentadiene among commercially available cyclopentadiene compounds.
Sr(PrMe4Cp)2 is disclosed in European Patent Application Laid-Open No. 1645656, and is registered under CAS No. 882296-98-2.
In “MOCVD & CVD Precursors”, Strem, 1999, CVD11/99, p. 22, a compound obtained by adding 1,2-dimethoxyethane (CH3OC2H4OCH3; hereinafter, referred to as DME) to Sr(PrMe4Cp)2 is described.
However, European Patent Application Laid-Open No. 1645656 describes only that an InAs film is grown at 600° C. by MOCVD using trimethylindium and monoethylarsine as raw materials in the presence of acatalytic amount (<0.25 mol %) of Sr(PrMe4Cp)2 in a table in Example 7. A trace amount of Sr(PrMe4Cp)2 is added as a catalyst and Sr is not substantially contained in the film. In addition, it does not describe about preparation and properties of Sr(PrMe4Cp)2.
“MOCVD & CVD Precursors”, Strem, 1999, CVD11/99, p. 22, also does not describe Sr(PrMe4Cp)2 containing no ethers added thereto.
Cyclopentadienyl-based strontium compounds are prepared from compounds containing ethers added thereto, by removing the ethers. These compounds are difficult to be prepared without a route through ether adducts, and ethers added thereto are difficult to be removed.
Therefore, Sr(PrMe4Cp)2 has thus not been clearly described about its preparation and physical properties. A film containing Sr as a main ingredient has also not been formed from Sr(PrMe4Cp)2 as a raw material. In preparation of Sr(PrMe4Cp)2, key issues are a type of ether compound to be used and a method of removing ethers.
Furthermore, when Na is mingled only slightly in a semiconductor thin film forming material, an electric field at a semiconductor interface becomes irregular and a conductive thin film is corroded to largely damage the semiconductor characteristics; accordingly, a Na concentration in an organic strontium complex is demanded to be as near zero as possible. K as well is demanded to be as near zero as possible.